We recently made the surprising discovery that mitochondria from retinal ganglion cell axons are passed onto neighboring astrocytes for degradation in the optic nerve head of normal mice. We present evidence that this pathway is conserved in species ranging from frogs to primates, and that it likely occurs widely throughout the mouse nervous system. Here, we propose to test the hypothesis that mitochondria transcellular degradation, or transmitophagy, is a common mechanism in the central nervous system through which normally-aged and stress-damaged neuronal mitochondria can be degraded. In addition, we will test the hypothesis that gamma- synuclein from axons plays a critical role in determining whether axonal mitochondria are degraded by the previously known cell-autonomous autophagy pathway, or rather by this newly discovered transcellular degradation pathway. The proposed studies will employ two vertebrate animal models, Mus musculus and Xenopus laevis, taking advantages of experimental approaches optimal in each, including live imaging in the latter. Both well established and highly novel methodologies, including new transgenic reagents of wide applicability, will be used to determine precisely where transmitophagy occurs, its cardinal structural features, and how long it takes. In addition, as a means to test the current working model, experiments will probe the cellular machinery underlying transmitophagy. Two accomplished laboratories with highly complementary expertise and technologies will come together to study transmitophagy at molecular and structural levels far beyond the capabilities of either laboratory, by using experimental designs that fully exploit the benefits of the collaborative approach. The proposed studies will comprehensively describe, and to some extent probe the mechanism of, a previously unknown degradation pathway in the central nervous system. Given the importance of both degradation pathways and mitochondria to nearly every neurodegenerative disorder, it is highly likely that transmitophagy will be found i the future to be a critically important pathway in a variety of neurodegenerative disorders. Thus, though the proposed experimental design aims to understand the basic cellular and molecular mechanisms involved in transmitophagy and is not directed at any disease, these studies are likely to a have a major impact on human health in the long-term.